<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background-color: red;
        }
        .box{
            background-color: green;
            margin: 10px 50px;
        }
    </style>
</head>
<body>
    <!-- div*3 -->
     <!-- div{$}*3 -->
    <div>1</div>
    <div class="box">2</div>
    <div>3</div>
</body>
</html>